Uniconazole enhances lodging resistance by increasing structural carbohydrate and sclerenchyma cell wall thickness of japonica rice (Oryza sativa L.) under shading stress

Lodging is a major limiting factor for grain yield and quality, and low light enhances lodging risk by increasing stem elongation and decreasing culm thickness. This study evaluated the effect of exogenous uniconazole (UCZ) on the basal stem physiological mechanism, cell structure, carbohydrates, and lignin-related enzymes in culm tissue under shading conditions. The results showed that shading stress significantly decreased grain yield and stem mechanical strength in 2020 and 2021. Compared with normal light (CK), shading rapidly decreased the breaking strength (M) and section modulus (SM). Exogenous UCZ increased culm wall thickness and improved lodging resistance. Uniconazole application also reduced plant height, gravity height, and second basal internode length by inducing a reduction in the cell length, and caused the cells to be densely arranged under both normal light and shading conditions. Scanning electron microscopy (SEM) revealed that the thickness of the sclerenchyma tissue cell wall increased under both conditions under the UCZ treatment, which induced an increase in the cell wall content. Uniconazole application significantly increased non-structural carbohydrate, lignin, cellulose, and hemicellulose contents in culm tissue. Moreover, UCZ application regulated lignin-related enzymes, such as phenylalanine ammonia-lyase (PAL), 4-coumarate: CoA ligase (4CL), cinnamate 4-hydroxylase (C4H), laccase (LAC) and peroxidase (POD). Thus, exogenous UCZ could enlarge sclerenchyma cell wall thickness, alter cell morphology in the stem, and increase stem strength by increasing carbohydrate content, including non-structural carbohydrates, cellulose, lignin, and hemicellulose, to enhance rice (Oryza sativa L.) lodging resistance under shading conditions.